Technical Abstract: Animals have the ability to change their behavior (behavioral plasticity) and physiology (physiological plasticity) based on the costs and benefits in order to ‘fit’ their environment (adaptation). Through natural selection, the population preserves and accumulates traits that are beneficial and rejects those that are bad in their prevailing environments. The surviving populations are able to contribute more genes associated with traits for increased fitness to subsequent generations. Natural selection is slow but constant; accumulating over multiple generations, and generally the animals’ change is silent and/or undetectable at a given point in history. Chickens were domesticated from the wild red jungle fowl. The principle of domestication of animals by humans is similar to that of natural selection: selecting the best animals with the highest survivability and reproducibility (artificial selection). Compared to natural selection, the process of artificial selection is motivated by human needs and acts more rapidly with more visible results over a short time period. This process has been further accelerated following the development of current breeding programs and the emergence of specialized breeding companies. A laying hen, for example, produces more than 300 hundred eggs a year, whereas a jungle fowl lays 4-6 eggs in a year. During the domestication process, chickens retained their capability to adapt their housing environments, which is usually achieved by genetic changes occurring during each generation and over generations. Genes control animals’ behavioral, physiological, immunological, and psychological responses to stressors, including environmental stimulations. Genetic improvement of animals, with the discovery of genomic sequences, will speed up breeding programs and has the potential to be used very successfully in selecting animals with high production efficiency and optimal welfare, resulting from resistance to stress, disease or both.